I've seen this happen in several versions 1.17 and up. When mobs are enclosed with fences on top of which one or more carpets are placed, they will occasionally phase through the fence. I can't push the mob to recreate this, but I have occasionally caught the mobs be in the middle of a fence with a carpet on top. I'm not sure if this is related to chunk loading or a collision issue with the carpet fence, but it seems to be getting worse.